CHAPTER IV 

 CELL DIVISION 



One of the most important biological questions in the several decades 

 following the enunciation of the cell theory by Schleiden and Schwann 

 in 1838 and 1839 had to do with the origin of cells. The founders of the 

 theory believed that cells might arise either by a process of division of a 

 preexisting cell, or by "free cell formation." In the latter case, cells 

 were thought to crystallize out, as it were, from a formative or nutritive 

 substance, the " cytoblastema. " The latter method was considered 

 to be the usual one. By 1855, however, biologists had arrived at the 

 conclusion that cells arose only from preexisting cells and this doctrine 

 was summed up in that year by Virchow, the famous pathologist, in the 

 words, " omnis cellula e cellula." 



Discovery of Cell Division. — While the origin of cells from cells was 

 thus early established, the mechanism by which cells originated from other 

 cells was not known until in the seventies and eighties of the last century. 

 This apparent slowness was probably due to the fact that investigators 

 were concerned in verifying the cell theory. It was also due in part to 

 the want of good lenses, in part to the fact that staining methods were 

 in their infancy, and that no accurate method of section cutting had been 

 devised. These early investigators of cell phenomena had frequently 

 to improve methods or devise entirely new ones and to design new ap- 

 paratus for their work. They labored under great difficulties. Remak 

 in 1855 and 1858 proposed a scheme of cell division which much resembled 

 the method described below under the head of amitosis. Remak's 

 scheme was essentially as follows. The nucleolus divides, and there 

 follows division of the nucleus into two parts, each containing a nucleolus. 

 The division of the nucleus is in turn followed by a progressive con- 

 striction about the middle of the cell, which is thus pinched in two. 

 This scheme was accepted as correct for some years, but investigators 

 at times noted that the process was not quite so simple as Remak out- 

 lined it. Schneider in 1873, followed in quick succession by a number of 

 others, made discoveries concerning cell division which showed that the 

 division of the cell was usually a complicated process, but that there was 

 also a less frequent method which did not differ greatly from Remak's 

 scheme. 



Nomenclature. — The complicated method has received several names 

 which have come into general use, namely, karyokinesis, jnitosis, and 



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